This invention relates to an apparatus for monitoring the winding of sliver (wrap formation) on a roll of a sliver guiding roll pair in a fiber processing machine, particularly a drawing frame. The apparatus includes a pressure roll and a driving roll as well as a pressing mechanism for pressing the pressure roll against the driving roll. Further, switching means are provided for deenergizing the drive of the driving roll when wrap formation occurs about either roll. There is further provided a device which accommodates the pressing mechanism and the pressure roll and which also serves for lifting the pressure roll off the driving roll.
In a known apparatus of the above-outlined type a pressure-roll holder is mounted by a bolt on a pivotal yoke. For exerting a pressure on the pressure-roll bearing, a pressure piston is provided in a guide bearing in the pressure-roll holder. A pressing lever exerts a pressure on the guide bearing by means of a compression spring situated between the pressing lever and the piston. The piston has an adjustable shifting element which may be immobilized by a setscrew and which is provided with a shifting cam. The shifting element carries an adjustable contact ring which may be immobilized by a setscrew. To the pivotal yoke a guide bar is secured by means of a carrier element made of an insulating material. The guide bar serves for receiving a contact sleeve which is slidably arranged on the guide bar such that the friction between the guide bar and the contact sleeve does not allow the latter to slide by virtue of its own weight. On the contact sleeve a contact cam is provided which is situated between the shifting cam and the contact ring. The contact sleeve and the contact ring are connected to opposite electric poles of an electric control. If sliver begins to be wound on a pressure roll or driving roll, the pressure roll is displaced against the resistance of the piston by the cooperating driving roll so that the piston is shifted upwardly until the switching element contacts the switching sleeve. In this manner, a switching function is initiated, resulting in an immediate stoppage of the operation of the drawing frame. The distance between the switching element and the contact edge corresponds to the switching stroke of the piston. This distance also corresponds to the thickness of the sliver winding on the roll. It is therefore apparent that the distance is chosen to be as small as possible to ensure that the drawing frame is brought to a standstill as rapidly as possible when wrap formation starts. It follows that the switching voltage must not be high to securely avoid arc generation between the switching element and the switching sleeve even in case of very small distances. If, because of an operational reason, the cylindrical working face of the pressure roll has to be re-ground, the diameter of the roll will necessarily be reduced. The pivotal yoke, however, is fixed in the same operational position independently from such a diameter decrease so that the piston, when pressure is exerted against the bearing, automatically executes a follow-up shift one-half the diametrical difference of the re-ground pressure roll. In such a follow-up adjustment the switching sleeve is also automatically adjusted to the same extent by means of the lid-like closure member. In this manner the distance is automatically shifted to the desired value upon each follow-up grinding of the pressure roll. Before the pivotal yoke is lifted off its abutment, the operating person, by means of a handle, releases the pressing lever so that the spring is relaxed. If the friction between the piston and the guide bearing is selected such that the piston, after lifting the pivot yoke, is not moved downwardly by its own weight, then the shifting cam, together with the contact sleeve, again attains its operational position upon re-positioning the pressing lever. If, however, the pressure roll has to be re-ground, resulting in a reduced roll diameter, the shifting cam displaces the contact sleeve downwardly into its operational position upon positioning the pressing lever until the pressure roll lies on the driving roll. In this manner, the distance between the contact cam and the contact ring is maintained in its original magnitude.
The above-outlined conventional apparatus requires substantial technical and constructional outlay for several reasons. For each roll pair a separate monitoring device is provided, that is, for the drawing frame a plurality of monitoring devices are necessary. It is an additional disadvantage that each individual monitoring device requires a substantial structural and installational outlay. It is a further drawback that in each instance a plurality of distances between the switching element and the switching sleeve have to be accurately set in order to prevent electric arcing even in case of very small distances.